1. Field of the Invention
The present invention relates generally to monitoring systems, and more particularly to a tag for use with a remote monitoring system for determining the physical whereabouts of a tagged object or person. More specifically, the invention concerns a tag having improved means for ensuring that removal of the tag is detected and signalled to the monitoring system. Particular utility for the present invention is found in the area of monitoring the whereabouts of mental patients, individuals suffering from senile dementia, and children, although other utilities are also contemplated, including other personnel and object monitoring applications.
2. Brief Description of Related Prior Art
Many devices presently exist for monitoring the physical location of objects and individuals. Many such systems use a tag secured to the object or individual whose location is to be monitored for transmitting radio-frequency signals to a remote monitoring system. The remote system uses the signals transmitted by the tag to determine the whereabouts of the object or individual carrying the tag.
One prior art monitoring system is disclosed in U.S. Pat. No. 4,885,571 to Pauley et al. In Pauley et al., the tag is worn by an individual being monitored, preferably on the individual's ankle or leg where it can be easily concealed by the individual's clothing. The tag is self-contained and sealed. The tag contains electronic circuits for periodically generating identification signals including an identification code. The identification signal modulates a stable radio-frequency signal that is transmitted in bursts of data words to a receiver associated with a field monitoring device (FMD) located at the area of confinement of the arrestee. The FMD randomly establishes communication with a central processing unit (CPU) located at a central monitoring location. The identification code also includes information indicating that an attempt has been made to remove the tag from the individual. The tag is held in place near the skin of the wearer by a conductive strap that wraps around the leg or other limb of the wearer. Two capacitive electrodes, one of which is realized by the strap, function as plates of a capacitor, with the body flesh of the wearer serving as the dielectric material of the capacitor. Thus, the two electrodes provide electrostatic coupling through the body mass of the wearer. By monitoring an alternating signal coupled from one capacitive electrode to the other, a determination can be made as to whether the tag has remained near the body flesh of the wearer. Additionally, a signal is passed through the conductive strap to permit a determination to be made as to whether the strap as been broken. The periodic signals transmitted by the tag permit the monitoring system to determine whether the person being monitored is within the area being monitored.
Thus, the tag disclosed in Pauley et al. utilizes a capacitive coupling to transmit electrical signals through the wearer's body to monitor whether the tag is being held near the wearer's skin or has been removed therefrom. Although this technique is somewhat effective in determining whether the tag is secured to the individual or object being monitored, it suffers from certain disadvantages. For example, some mental patients, individuals suffering from senile dementias, and children have an irrational fear of electrical energy and are terrified of having electrical energy transmitted through their bodies. Such individuals usually will not willingly consent to wear the tag disclosed in Pauley et al. and may become violent and/or agitated if they are forced to wear it, and/or will remove the tag when they are left unattended.
Additionally, transmission of electric energy through certain types of magnetic recording media (e.g., magnetic tape, disks, etc.) can damage or destroy the information stored on the media. This renders the capacitive monitoring tag of Pauley et al. generally undesirable for use in monitoring the location of such media.
Other prior art tag monitoring systems are disclosed in U.S. Pat. No. 5,285,194 to Ferguson; U.S. Pat. No. 5,245,317 to Chidley et al.; U.S. Pat. No. 5,196,825 to Young; U.S. Pat. No. 5,014,040 to Weaver et al.; U.S. Pat. No. 4,952,913 to Pauley et al.; U.S. Pat. No. 4,918,432 to Pauley et al.; and U.S. Pat. No. 4,471,345 to Barrett, Jr. The tag monitoring systems disclosed in these patents suffer from the aforesaid and/or other disadvantages.